[netmod] RFC8526 and 'derived-from-or-self(datastore, "ds:operational")'
Jernej Tuljak <jernej.tuljak@mg-soft.si> Mon, 12 August 2019 21:50 UTC
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From: Jernej Tuljak <jernej.tuljak@mg-soft.si>
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Date: Mon, 12 Aug 2019 15:04:54 +0200
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Subject: [netmod] RFC8526 and 'derived-from-or-self(datastore, "ds:operational")'
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Hi, I'm trying to determine who is at fault, our YANG validator implementation or the module ietf-netconf-nmda in RFC8526 . There is a "when" expression at line 193 within this module when 'derived-from-or-self(datastore, "ds:operational") '; which is under this pseudo-branch within the module: / get-data ("rpc") / input ("input") / origin-filters ("choice"). Our implementation claims that the initial context node of the expression does not contain a "datastore" child, resulting in a warning for this expression, which seemed like an obvious bug at first, but does not anymore. The accessible tree for this expression is built using the following bullet in Section 6.4.1: o If the XPath expression is defined in a substatement to an "input" statement in an "rpc" or "action" statement, the accessible tree is the RPC or action operation instance, all state data in the server, and the running configuration datastore. The root node has top-level data nodes in all modules as children. Additionally, for an RPC, the root node also has the node representing the RPC operation being defined as a child. The node representing the operation being defined has the operation's input parameters as children. "origin-filters" for which the "when" expression is defined is clearly a substatement to an "input" statement in an "rpc" statement. Pseudo- accessible tree below (matches what our implementation does). <root> +- get-data +- datastore +- // other data node siblings of datastore +- // top-level data nodes of all modules The initial context node is determined by using the following bullet in Section 7.21.5: o If the "when" statement is a child of a "uses", "choice", or "case" statement, then the context node is the closest ancestor node to the node with the "when" statement that is also a data node. If no such node exists, the context node is the root node. The accessible tree is tentatively altered during the processing of the XPath expression by removing all instances (if any) of the nodes added by the "uses", "choice", or "case" statement. The "when" statement in question is clearly a child of a "choice" statement without a data node ancestor. Neither "input" nor "rpc" represent data nodes, as per Terminology section: o data node: A node in the schema tree that can be instantiated in a data tree. One of container, leaf, leaf-list, list, anydata, and anyxml. To our implementation this means that the initial context node is the root node of the accessible tree, not the node that represents the "rpc". Therefore the expression is attempting to refer to a node that is undefined. It is expecting when 'derived-from-or-self(get-data/datastore, "ds:operational") '; or something along those lines instead. Either way, something doesn't add up between RFC7950 and RFC8526. There is a suspicious verified RFC7950 errata (https://www.rfc-editor.org/errata/eid4794) dealing with initial context nodes under augments, but there's nothing for the UCC ("uses", "choice", "case") corner case. Perhaps there should be? Should Section 7.21.5 say NEW: o If the "when" statement is a child of a "uses", "choice", or "case" statement, then the context node is the closest ancestor node to the node with the "when" statement that is also a data node, rpc, action or notification. If no such node exists, the context node is the root node. The accessible tree is tentatively altered during the processing of the XPath expression by removing all instances (if any) of the nodes added by the "uses", "choice", or "case" statement. OLD: o If the "when" statement is a child of a "uses", "choice", or "case" statement, then the context node is the closest ancestor node to the node with the "when" statement that is also a data node. If no such node exists, the context node is the root node. The accessible tree is tentatively altered during the processing of the XPath expression by removing all instances (if any) of the nodes added by the "uses", "choice", or "case" statement. ? Jernej
- [netmod] RFC8526 and 'derived-from-or-self(datast… Jernej Tuljak
- Re: [netmod] RFC8526 and 'derived-from-or-self(da… Martin Bjorklund
- Re: [netmod] RFC8526 and 'derived-from-or-self(da… Ladislav Lhotka